Canon 1D Mark III advantages over Canon 5DS

Longer stills battery life

Cameras with longer battery life can take more photos before exhausting their batteries.

Special note: The measurement standard for battery life stipulates that if a camera has an internal flash, it must be used for 50% of photos taken. For this reason, comparisons of one camera with an internal flash to another without will not be comparable

2200
vs
700 shots

Capture more photos

Higher max flash sync

1/300
vs
1/200 sec

Reduce the effect of ambient light in flash shots

Less shutter lag

0.09
vs
0.27 sec

Focus and take a photo quickly (wide angle)

Faster JPEG shooting

9.9 fps
vs
4.8 fps

Faster JPEG shooting (burst mode)

Canon 5DS advantages over Canon 1D Mark III

Larger sensor

35mm
vs
APS-H

More sensor area. Bigger is (generally) better.

Higher effective ISO

Higher effective ISO

Maximum effective ISO is an estimate of the highest sensitivity at which a camera can capture excellent quality photos.

Cameras with higher effective ISO will be better choices for indoor photography, night shooting, and indoor sports photography, especially if you intend to make large prints.